Many human premature infants develop bronchopulmonary dysplasia (BPD), and some studies suggest a relationship between premature retinol levels and the incidence or severity of BPD. However, accurate assessment or evaluation of a possible "conditional vitamin A deficiency" in this patient population has not been accomplished. Vitamin A is essential for normal epithelial cell growth and differentiation. In lung, vitamin A deficiency is characterized by a loss of cilia and the development of squamous metaplasia in the respiratory tract which are similar to changes found in BPD. The overall objective here is to clarify the role of vitamin A in the pathogenesis and repair of BPD First, we will assess the vitamin A (retinol) status of low birthweight (less than 1500 grams) premature infants with an intramuscular Relative Dose Response (IM-RDR) test. Optimal conditions will first be established in the vitamin-A-deficient rat model. Then, the IM-RDR Will be used in premature infants of birthweight less than 1500 grams to see if its estimate of the vitamin A status at day 0 and day 28 is predictive or unrelated to the chances of having BPD. The IM-RDR will be validated with liver biopsy samples when infant deaths occur. Serum RBP levels will be done simultaneously with the IM-RDR. The IM-RDR (for retinol and RBP) will be compared in gestational age corrected populations without lung disease at day 28 to those with BPD. BPD will be diagnosed by established criteria. If the IM-RDR can be used to predict the chances of BPD, we will study BPD and vitamin A supplementation. Second, we will study how vitamin A might relate to BPD on a biochemical level through its action on the alveolar type II epithelial cell. We will demonstrate that the isolated type II cell takes up and utilizes retinol and retinoic acid. This will be done by showing: a) uptake and metabolism of retinol to retinoic acid; b) presence of cellular-binding protein for retinol (CRBP); c) presence of retinoic acid nuclear receptors (RARs), quantitation of expression. Other experiments will demonstrate biochemically that vitamin A maintains the integrity of the differentiated state of type II epithelial cells in culture. These include: a) synthesis of phospholipids in control and vitamin-A-deficient cells; b) measurement of the status of type II cell markers in cells of control and deficient animals, and the effect of vitamin A (retinoic acid); c) assess changes in key enzyme activities that are closely associated with proliferation/differentiation that occur in control and deficient cells, and determine their response to retinoic acid (e.g., Ornithine Decarboxylase and Transglutaminase). Finally, we will study how the interaction of oxygen exposure and retinol deficiency alters the populations of type II/I cells in neonatal rat lungs. This will be done using cell labeling and morphometry.